1. Field of the Invention
This invention relates generally to the field of portable paging systems and in particular to a portable paging device in which the signal processing is performed by a decoder operating in real time to provide multiple scheme decoding and energy-saving routines.
2. Description of the Prior Art
Pagers are generally small, portable, battery-operated radio receivers designed to be carried on the person of the user in a pocket or clipped to a belt. A typical user might be a medical doctor or maintenance person. The technique used in paging systems is referred to as selective calling in which transmissions intended for a particular receiver will cause a response only in that receiver. The selective calling capability is achieved by assigning to each pager one or more unique address codes which are encoded and modulated on the carrier wave. Each pager includes a decoding portion which is designed to cause a response only to its assigned address codes. If one of the assigned address codes is detected, the pager is activated to produce a corresponding alert signal. Normally, none of the other pagers in the communications system will be activated by that transmission unless the system has a group call operation.
Pager communication systems are well known in the art and operate with many types of coding schemes including single or multi-tone signals, selected combination of tone and voice signals and digitally encoded signals. Typically there are wide area paging services capable of transmitting throughout a metropolitan area and local paging services which transmit in a much smaller service area as, for example, a specific building complex or other facility. The range of operation of the various systems is generally determined by the power of the transmitter used.
If a pager carrier is required to receive signals within the boundaries of several facilities, he could require a separate local pager for each such facility. The occurrence of such problems can be frequent, as is the case with doctors who may be on the staff of several hospitals and are required to visit patients in several hospital locations. Moreover, changing location, such as traveling to a nearby city having a different wide area service system, would render an original wide area pager and all of the several local pagers useless and require an entirely new set of pagers. Thus, it would be advantageous to have a paging device which can function to provide decoding and alert signaling for more than one pager communications system. Then the difficulty of changing pager addresses on a specific device or physcially changing pagers or altering circuitry to change address codes when a pager is carried from one message service area to another could be eliminated.
While there are several tone only and tone and voice signal decoding systems, the decoding of such information is generally not as difficult or as demanding as decoding those systems which employ digitally encoded signals. Such digital systems are capable of a high volume of messages transmitted per unit time of the faster information rate they can transmit for addressing individual paging devices.
The digital information which is sent out in a digital communications system normally comprises a binary signal train which includes level transitions from a first level to a second level. These levels indicate the distinction between a binary .0. and a binary 1. There must also be a specific time duration which corresponds to a bit interval. The non-return-to-zero (NRZ) binary coding system generally used does not delineate bit intervals. It therefore is necessary to ascertain when an information bit begins or ends and this applies to both synchronous and asychronous systems. Because of the variability of the nature of the information in an NRZ serial binary pulse train, several bit intervals could pass without the occurrence of a transition in the signal levels. Thus it is generally not obvious simply from the information received at a receiver precisely when a bit interval begins or ends.
There are two general types of digital systems and they are distinguishable by the time relationships between transmitter and receiver. In synchronous systems, the transmitter and receiver must be in synchronization so that bit address sequences and frames delineating information words can be correctly decoded. By contrast, asynchronous systems normally decode a preliminary information word which constitutes part of an address and, when identified, causes the establishment of a time window in anticipation of the subsequent detection of digital information related to the remaining portion of the address of an individual paging device. With either synchronous or asynchronous systems, the difficulties of providing decoding for more than one encoding system are complex. Moreover, the presence of a multi-system capability in a pager demands sort of prioritizing to insure proper function when more than one system can be detected by the pager.
The prior art signal processing circuitry for pager communication devices generally comprise custom integrated circuits or custom hybrid circuits which perform the specific high speed decoding functions for a single signal decoding system. A multi-system pager by utilizing the present IC and hybrid decoding technology is prohibitive from the view of cost, design complexity and power consumption. In addition, there exist present and proposed pager communications systems which, because of total unit market volume, could not support the initial tooling and engineering costs associated with the development of the custom circuitry presently required and are therefore prevented from being established. However, the economy offered by a multi-system pager would make it possible to profitably manufacture relatively small numbers of single system pagers by adapting a high unit volume, multi-system pager to decode one specific decoding scheme.
A critical requirement for any signal decoder for a paging system is that it must process signals in real time so that decoding does not fall so far behind that information is lost. The concept of real time processing is well known in the art and is associated with the actual time during which physical events take place. With respect to real time signal processing by a decoder, the operations performed by the decoder which relate to a detection and decoding process must be sufficiently rapid so that the results obtained are useful in controlling the device without the loss of information. Thus the operational speed of a multi-system decoder which is to perform digital signal processing is directly related to the information rate at which the pager communication system operates. The digital information transmission rate may vary from system to system and it is clear that faster rates are preferred because more information may be handled by an entire system in a given unit of time. Clearly, the faster the information rate the greater the processing speed demand on a multi-system decoder.
The high clock rate for a multi-system decoder performing signal processing in real time increases both the voltage required and power consumed by the system and thus can adversely affect the operating economics of a pager and could affect both the size and weight of the device. Just as it is a disadvantage to carry several different pagers around, it is equally disadvantageous to carry a bulky or heavy power pack to provide the power for the operation of a paging device. In general, the greater the power consumption, the larger the mass of the batteries which must be carried to provide for the operation. Indeed if the power drain is too great, fresh primary cells might be required for every few hours of operation. This could even create a situation in which operating batteries and several fresh cells would be carried by the person using the pager to ensure continuous operation. Because the pager is for individual portable operation, it is imperative that battery energy be conserved to provide a useful operational life for multi-system signal decoding.